Generally, a transparent conductive film is used for plasma display panels (PDPs), liquid crystal displays (LCDs), light emitting diodes (LEDs), organic electroluminescent devices (OELDs), touch panels, solar cells, etc.
Because the transparent conductive film has high conductivity (e.g., sheet resistance not higher than 1×103 Ω/sq) and high transmittance in the visible light range, it may be used as the electrode of not only solar cells, LCDs and PDPs but also of various light-receiving devices and light-emitting devices, and as well, as a transparent electromagnetic wave blocking unit, including an antistatic film or an electromagnetic wave blocking film, for use in automobile window glass or building window glass, and a transparent heating unit, including an infrared reflection film or a freezing showcase.
The transparent conductive film includes antimony or fluorine-doped tin oxide (SnO2) films, aluminum or potassium-doped zinc oxide (ZnO) films, tin-doped indium oxide (In2O3) films, etc.
In particular, the tin-doped indium oxide film, that is, an In2O3—Sn based film, is called an ITO (Indium Tin Oxide) film, and is frequently used because it facilitates the formation of a low resistance film. Although ITO, having superior general properties, is mainly applied to the process to date, indium oxide (In2O3) is produced as a by-product in a zinc (Zn) mine, and problems of instable supply and unmet demand arise. Further, the ITO film is not flexible and thus cannot be used as a flexible material for a polymer substrate, and furthermore, because it is manufactured under conditions of high temperature and high pressure, the production cost thereof is undesirably high.
In order to obtain a flexible display, a conductive polymer may be applied on the upper surface of a polymer substrate, but the film thus obtained has low electrical conductivity or is opaque, and therefore, the use thereof is limited.
With the aim of solving the above problems, techniques for applying carbon nanotubes on various types of substrates are widely studied these days. The carbon nanotubes have electrical resistance of 10−4 Ωcm to thus exhibit electrical conductivity similar to that of metal, and have a surface area at least 1000 times larger than bulk materials and a length thousands of times longer than the outer diameter thereof, and thus are considered to be an ideal material for realizing desired conductivity. Moreover, the carbon nanotubes may be improved in adhesive force to a substrate through surface functionalization thereof. In particular, because the carbon nanotubes may be applied to the flexible substrate, the uses thereof are expected to be numerous.
A conventional technique using carbon nanotubes, entitled “coating containing carbon nanotubes” (Korean Unexamined Patent Publication No. 10-2004-0030553), is disclosed. In the conventional technique, because only carbon nanotubes having an outer diameter of 3.5 nm should be used, in consideration of the dispersibility and electrical conductivity thereof, there is a limitation in the use of the material. Also, when the coating is prepared, the properties, including dispersibility and adhesiveness, of the carbon nanotubes are gradually decreased over time.